The present invention relates to an improvement of an ISDN user-network interface which implements various kinds of communication including a circuit switching type communication, and/or a packet type communicaton between a communication network and various kinds of terminal apparatuses installed in a subscriber through a subscriber line.
FIG. 1 shows a block diagram of a user-network interface defined by the D-channel protocol (I-400 series recommendations) recommended by CCITT in 1984. In the figure, the symbol NT(11) is a network termination unit which terminates both a subscriber line and a subscriber local bus, and coupling the same with each other, TE.sub.1, TE.sub.2. . . TE.sub.i (12) are terminal equipment LS(13) is a subscriber line exchange system. A subscriber line 14 is provided between the exchange system LS and the termination unit NT. The line 14 comprises two B-channels 14a in which bothway communication signals are transmitted, and a D-channel 14b in which a call control signal and packet type communication signals are transmitted. Each terminal equipment TE.sub.i is coupled with the subscriber local bus 15 which is terminated by the termination unit NT. Two B channels 15a and a D-channel 15b in a subscriber local bus are provided between the termination unit NT and the terminal apparatuses TE. Further, an echo channel (called E-channel) in the direction from the termination unit NT to the terminal apparatuses TE is provided in a subscriber local bus so that the D-channel signal from the terminal equipment TE to the termination unit NT is offset by the termination nation unit NT to the echo channel. The E channel is used to prevent an access collision by more than two terminal equipment, that is to say, each terminal equipment TE.sub.i monitors the echo channel, and when the terminal equipment TE.sub.i finds that the D-channel from the terminal equipment TE.sub.i to the termination unit NT 15.sub.b1 is available, the terminal equipment TE.sub.i sends a signal to that D-channel 15.sub.b1. As the signal on the D-channel 15.sub.b1 is offset in the termination unit NT, and appears in the echo line, the terminal equipment TE.sub.i tests if the signal on the echo line is the same as the signal which the terminal equipment TE.sub.i itself sent to the D-channel 15.sub.b1. When those signals differ from each other, the terminal equipment TE.sub.i stops sending a signal.
A signal is sent in a frame which has a flag, address part, control part for recognizing what kind of frame it is, information part, and frame check sequence part for error detection. The address part consists of a service access point identifier (SAPI) which shows the kind of service (for instance, transfer of call control signal, or transmission of packet type communication signal), and terminal endpoint identifier (TEI). When a frame which has a call control signal, and/or packet type communication signal is transmitted to the D-channel, it is acknowledged that the signal sent is received correctly in the exchange system LS for every frame, or for more than one frame. Another method for signal transfer is provided in which no information acknowledgement is carried. This method is mainly used for a local signal. The information for indicating whether or not the acknowledgement is sent, is included in the control part of a frame.
However, according to the D-channel protocol recommended in 1984, there is the disadvantage that a communication among terminal equipment in a subscriber is not possible, although communication between a terminal equipment in a subscriber and the end terminal equipment in another subscriber or an information processing unit which locates in a communication network is possible.
Further, there is the disadvantage that it is impossible to switch a signal between terminal equipments without losing a reception signal from far end of the communication line. Thus, the system of FIG. 1 has the disadvantage that it can not use various kinds of terminal apparatuses effectively.
Further, there is the disadvantage that terminal equipment in a subscriber just communicates with terminal equipment in another subscriber independently of the terminal equipment in the same subscriber and that several terminal equipment in the same subscriber can not correspond a communication with other subscribers cooperatively.
Thus, each function provided by several of the terminal equipment in the same subscriber can not be used effectively.